The present invention relates to a process for making a fluoropolymer that has nitrile end groups.
Fluoropolymers, i.e. polymers having a fluorinated backbone, have long been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, and UV-stability etc. Various examples are described in xe2x80x9cModern Fluoropolymersxe2x80x9d, edited by John Scheirs, Wiley Science 1997.
The known fluoropolymers include fluoroelastomers and fluorothermoplastics. Such fluoropolymers are generally copolymers of a gaseous fluorinated olefin such as tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE) and/or vinylidene fluoride (VDF) with one or more comonomers such as hexafluoropropylene (HFP) or perfluorovinyl ethers (PVE) or non-fluorinated olefins such as ethylene (E) and propylene (P). Examples of fluoroelastomers include copolymers of TFE and PVE and copolymers of VDF and HFP. The fluoroelastomers may also contain cure site components so that they may be cured if desired. Suitable cure site comonomers include nitrile containing cure site monomers, such as nitrile containing olefins and nitrile containing fluorinated vinyl ethers. See, for example, WO 01/27194. EP 1 211 265 discloses that the use of nitrile containing comonomers, such as nitrile containing PVE, is rather expensive. The nitrile group-containing fluoroelastomer is said to be unstable and the properties of the cured product are said to be insufficient. Accordingly, EP 1 211 265 proposes to prepare nitrile group containing fluoroelastomers by converting amide group containing fluoroelastomers, in the presence of a dehydrating agent. This process has the disadvantage that an additional process step is needed which involves supplying additional energy. Accordingly, such a process increases the cost of manufacturing and causes inconveniences in the manufacturing process.
EP 868447 discloses the use of certain chain transfer agents (Ixe2x80x94(CF2)nxe2x80x94CN) to introduce nitrile endgroups; however the compounds used are expensive, toxic and not easy to handle.
Finding another method of introducing nitrile groups into a fluoropolymer is desirable. Such method should be cost effective and allow for a convenient manufacturing process. Desirably, cured fluoropolymer products and in particular fluoroelastomers can be prepared from such fluoropolymers. Preferably, such cured fluoropolymers and elastomers have good physical and mechanical properties including for example heat resistance and good processibility.
In accordance with the present invention, when the present fluoropolymer is produced by free radical polymerization of fluorinated monomers in the presence of a nitrile group containing salt or a nitrile group containing pseudohalogen compound, a fluoropolymer results that has nitrile end groups. Such nitrile groups may be used in a so-called nitrile cure reaction as is known in the art to produce cured fluoropolymers and in particular to produce fluoroelastomers. Further the resulting fluoropolymer may have improved processibility. By the term xe2x80x9cfluoropolymerxe2x80x9d in connection with this invention is meant a polymer that has a fluorinated backbone, i.e. a partially or fully fluorinated backbone.
Generally, the amount of the nitrile group containing salt or nitrile group containing pseudohalogen compound is selected relative to the amount of initiator used to initiate the polymerization. Conveniently, the ratio of the amount of nitrile group containing salt or pseudohalogen compound to initiator is selected such that the amount of nitrile end groups in the resulting fluoropolymer according to Fourier Transform IR-measurements as set out in the examples is at least 0.0002, preferably at least 0.0004 and most preferably at least 0.001.
Still further, the present invention also provides a fluoropolymer composition for making a fluoroelastomer, comprising the above fluoropolymer and a cure composition.